Background: Decisions by dentists to administer antibiotic prophylaxis to prevent infectious complications in patients involves professional risk assessment. While recommendations for rational use have been published, several studies have shown that dentists have low adherence to these recommendations. Objective: To examine general dental practitioners’ (GDPs’) assessments of the risk of complications if not administering antibiotic prophylaxis in connection with dental procedures in patients with specific medical conditions. Methods: Postal questionnaires in combination with telephone interviews. Risk assessments were made using visual analogue scales (VAS), where zero represented “insignificant risk” and 100 represented a “very significant risk”. Results: Response rate: 51%. The mean risk assessments were higher for GDPs who administered antibiotics (mean = 54, SD = 23, range 26–72 mm on the VAS) than those who did not (mean = 14, SD = 12, range 7–31 mm) (P < 0.05). Generally, GDPs made higher risk assessments for patients with medical conditions that are included in recommendations than those with conditions that are not included. Overall, risk assessments were higher for tooth removal than for scaling or root canal treatment, even though the risk assessments should be considered equal for these interventions. Conclusions: GDPs’ risk assessments were rational but uninformed. They administered antibiotics in a manner that was consistent with their risk assessments. Their risk assessments, however, were overestimated. Inaccurate judgments of risk should not be expected to disappear in the presence of new information. To achieve change, clinicians must be motivated to improve behaviour and an evidence- based implementation strategy is required.